The general objective of the proposed studies is to characterize the molecular mechanisms underlying the proximal tubular transport of urate. The experimental approach will be to analyze urate transport pathways in plasma membrane vesicles separately isolated from the luminal and basolateral surfaces of the proximal tubular cell. In the first place, the mechanisms for urate transport across the luminal membrane will be further defined. Recent studies from my laboratory have identified in dog microvillus membrane vesicles an anion exchange pathway by which uphill urate absorption can be coupled to outwardly directed gradients of OH-, CL- and certain organic anions, such as lactate and PAH. The properties of this anion exchange pathway will be investigated including the stoichiometry of anion exchange, the relative affinities of various anions sharing the exchanger, and the effects of drugs known to alter renal urate excretion. In addition, the techniques and conceptual models developed on the basis of these studies in an experimental animal will then be applied to the study of urate transport in microvillus membrane vesicles isolated from the human renal cortex. Secondly, the mechanisms of urate transport across the basolateral cell surface will be evaluated. Using basolateral membrane vesicles isolated from the rabbit kndney, we will examine the possible coupling between urate flux and electrochemical gradients of Na+, K+, H+, (OH-), HCO3-, CL-, and organic anions. The kinetics, substrate specificity, and sensitivity to inhibitors will be analyzed for the coupled transport pathways that are identified. Again, the techniques and conceptual models developed on the basis of these studies in an experimental animal will then be applied to the study of urate transport in basolateral membrane vesicles isolated from the human renal cortex. The mechanisms of uphill anion transport elucidated by this project will be of general biolobical interest. Moreover, these studies should ultimately provide new insight into the subcellular mechanisms underlying bidirectional urate tranport in the human proximal tubule. Understanding these mechanisms is clearly relevant to the clinical problems of gout and nephrolithiasis.